As individuals age, their movement control performance generally deteriorates; in particular, their movements get slower and more variable. We have shown that in an aiming task much of the slowing can be attributed to how the elderly subjects structure their movements. In comparison to younger subjects, they typically execute aiming movements that contain a shorter movement. We have also established that even with extended practice, the elderly do not change the relationship between these sub-movements which suggests that these modifications to the movement structure reflect fundamental changes in the motor system. In this application through a series of experiments, we seek to understand why the elderly alter the structure of their movements. Four major hypotheses are examined that probe potential causes of this modification: increase muscle cocontraction, increased force variability, reduced visuo-proprioceptive calibration, and reduced coordination control. In each experiment a methodology is employed that divides aiming movements into primary and secondary sub-movements and investigates their relationship to predictions made from the four hypotheses. This experimental sequence is the first to comprehensively examine primary and secondary sub-movements in the elderly.